The backfilling of sewers, embankments, structures and utility trenches, for example electrical, gas and water conduits, has often created a problem in the past. This is particularly true where, for example, a pavement or other structure must be placed over a backfilled sewer. If a sewer line was installed by using, for example, an open trench method of construction and if poor soil conditions were present, the normal method of construction to obtain the desired compaction of the backfill material was to remove the original soil and replace it with sand, the original material or other specified backfill soil. In addition, it was often necessary to backfill in layers, often as small as three inch layers, and then mechanically compact these layers. While the results were satisfactory, this was a very expensive method of construction.
A similar problem was encountered if the soil bedding was so poor that it had to be removed and replaced with an adequate bedding material for the conduit. It is very important to have a proper bedding under conduits so that the resulting forces are equalized throughout the length of the conduit. For example, if rigid pipe is utilized as the conduit material, and if the bedding does not uniformly support the conduit throughout its length, there is a tendency for the conduit to shear or otherwise fail. In the case of flexible pipe, proper backfill is necessary to insure passive resistance pressure, otherwise the pipe will fail by excessive deflection.
To backfill the trench with, for example, a concrete is also not a proper solution. For example, if a sanitary sewer is being backfilled, and the backfill material is concrete, it is very difficult if not impossible to dig downwardly through the concrete to either repair a break in the line, install a lateral sewer connection into the main line, or make a house tap.
The present invention is directed to solving the above problems and consists of a controlled density fill material. The controlled density fill material, while including Portland cement as one of its constituents, is not a concrete as the term is normally used in the construction industry. A conventional concrete has an average concrete comparison 28-day compressive strength of 4000 psi while the controlled density fill material, according to the present invention, has a compressive strength preferably less than 1600 psi. In fact, when utilized in backfilling a sewer conduit, the preferred material, according to the present invention, has a compressive strength in the range of 200 psi.
On the other hand, the controlled density fill material, according to the present invention, is not a soil as that term is typically used in the construction industry. Rather than having to compact individual layers of the controlled density fill material, according to the present invention, one needs only to place the mix into the trench. The material, because it does not have the structural strength of concrete, may be re-excavated, for example, by using a trencher, in order to reach the pipe to make a repair or to make a connection for a future lateral line.
Furthermore, because the controlled density fill material does not have the rigid structural strength of concrete, it is also an excellent material for use in backfilling adjacent a structure, for example against a bridge abutment. It may also be placed in embankments in place of the customary earth compaction system of construction.